An extrusion die is used to extrude molten thermoplastic material into a relatively thin film or sheet. Conventional extrusion dies have a die cavity with the general shape of a coat hanger. These extrusion dies have been generally referred to as coat hanger dies. Typical coat hanger dies include an inlet, an inlet manifold, a generally triangular shaped preland channel, a final land channel, and a die exit or gap. In operation, molten polymer flows through the extrusion die from the inlet to the die exit. The size and shape of the different channels are selected to promote uniform polymer flow across the face of the die and produce a thin sheet or film having a desired thickness.
In practice, sheet manufacturers are often required to produce sheets of different thicknesses to meet various customer demands. For this reason, an extrusion die can have adjustable lip gap spacing. The distance between the lips of the extrusion die can be adjusted by the manufacturer to control the size of the die exit. In turn, this controls the thickness of the sheet exiting the die.
While adjustable lip gap spacing is helpful to provide manufacturers with processing flexibility, other processing challenges can arise when lip gap spacing is widened or narrowed on an extrusion die. The rheological characteristics of the molten polymer flowing through the extrusion die are a function not only of the lip gap spacing at the die exit but also the configuration of the flow pathways leading up to the die exit. For example, flow pathways leading up to the die exit, including the final land channel, are generally sized for a specific die exit size. This ensures good back pressure and flow characteristics through the extrusion die. With adjustable lip gap dies, manufacturers can size the flow pathways to be optimized for the middle die exit size between the smallest and largest adjustable opening sizes. But this compromises processing characteristics when operating at a smaller or larger die exit size, potentially resulting in product orientation and shrinkage issues.